The rabbit corpus luteum responds to estradiol, which is both the necessary and sufficient luteotropic hormone in this species. We propose to investigate the biochemical action of estrogen in the corpus luteum by determining if cyclic AMP and cyclic AMP-dependent protein kinase are involved in the steroidogenic response. These studies will be conducted in hypophysectomized rabbits to preclude the possible action of LH, which can be shown to acutely stimulate progesterone synthesis in the corpus luteum. The cause of luteal regression and termination of the luteal phase in the hypophysectomized rabbit will be investigated by determining if luteal estrogen receptor is gradually lost, thus causing a gradual loss in luteal responsiveness to the luteotropic hormone.